Modem wideband (broadband) wireless communication systems use a broadband link that is established in a spectrally efficient way by employing linear modulation schemes as QPSK, OQPSK and OFDM. The design of power amplifiers in the RF transmitters is challenging because the goal is to amplify a modulated RF signal with high peak-to-average while using minimum DC power consumption. Class AB amplifiers traditionally used for RF amplification are poorly suited to meet these goals.
A number of newer amplifiers, e.g., envelope tracking (ET) or envelope elimination and restoration (EER) power amplifiers can operate more efficiently in wideband communication systems. However, as the bandwidth of the communication signals becomes broader, the design of the supply modulator becomes the design bottleneck in such supply-modulated RF power amplifiers. The power supply modulator has therefore constrained the design of power supply modulated RF power amplifiers such as the ET, EER, wideband ET amplifiers, and average power ET step ET, also called high accuracy tracking (HAT) amplifiers.
United States published application 20090004981 discloses a method of improving the power efficiency of a digital transmitter for non-constant-amplitude modulation schemes. A bandwidth reduction provides slow form (i.e. reduced bandwidth) of envelope tracking (based on a narrower bandwidth distorted version of the envelope waveform) such that the switching regulator can use a lower switching rate. Pre-distortion of the RF input signal is used to compensate and in one embodiment a look up table is used to provide input amplitude and supply voltage indexed data to provide information for calculating the necessary pre-distortion, but calculation of look up table entries and maintenance of the same is not considered. Bandwidth reduction steps are accomplished based on a peak detector function that detects the local maxima points in the envelope signal and then provides smoothed waveform based upon the detected peaks.